trophic structure many early researchers quickly realized communities and systems were much to...
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Trophic Structure
• Many early researchers quickly realized communities and systems were much to complex to analyze easily
• Consequently many adopted a trophodynamic approach
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Trophic Structure
• Feeding relationships were identified and communities were divided accordingly
• Any problems with this approach?
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Trophic Structure
• Problems associated with trophic position (can be variable in both time & space), omnivory, and their relative position in the food web can change in time & space (how?)
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Trophic Structure
• However, energy does flow through a system and certainly offers the potential to classify organisms
• Producers: produce organic compounds from inorganic compounds
• Consumers: primary consumers feed directly upon producers or each other while secondary consumers feed on primary consumers…
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Trophic Structure
• Can you think of an example of a relatively simple food chain involving 3+ levels?
• We can add another important component to our trophic…
•
decomposers
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Trophic Structure
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Trophic Structure
• Why is it a pyramid?• Energy is lost at each trophic level
through metabolic and respiratory activity
• How big do you think a pyramid could get?
• Why is it limited to that length?
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Trophic Structure
• This representation is too simplistic (why?) and many organisms don’t fit neatly into a single level
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Trophic Structure
• While exact relationships within a trophic community may be difficult to discern, it is frequently more interesting to compare how different communities are structured
• E.g. number of levels, relative importance (top-down vs. bottom up)
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Trophic Structure
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Trophic Structure
• Such differences in trophic emphasis reflect different methods of opertation in the different communities and can be used to understand their functional roles
• If true, it may suggests the trophic organization of a community shows rather precise adaptation to that community’s function
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Trophic Structure
• This was demonstrated by Heatwole and Levins (1972) reanalyzing the data of Simberloff and Wilson (1969)
• S&W had ‘cleansed’ small mangrove islands in the FL Keys and monitored the subsequent colonization by arthropods
• What do you think they found?
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Trophic Structure
• However, H&L re-analyzed the original data in terms of trophic organization
• Counted number of species before and after defaunation in each trophic category (herbivores, scavengers, detritus feeders, wood borers, ants, predators and parasites)
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Trophic Structure
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Trophic Structure
• However, don’t get too excited…
• What might be a test of ‘random’
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Trophic Structure:energy flow
• Examining trophic relationships occurs when they are ‘static’
• Many studies have come to the conclusion that trophic systems are inherently unstable
• However, given enough energy passes through a system, almost all can become stablized
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Trophic Structure:energy flow
• Consider 4 aspects of energy flow– 1) amounts of energy– 2) efficiency of energy transfer– 3) rate (or speed) of flow– 4) nature of the associated nutrient cycle
(e.g. open or closed; sedimentary or non-)
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Trophic Structure:energy flow
• 1) Amount of energy…
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Trophic Structure:energy flow
• The total amount of energy captured within a system by the producers is known as gross primary productivity
• (GPP-respiratory cost) = Net PP
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Trophic Structure:energy flow
• 2) Efficiency of energy transfer• We already discussed losses
• In terms of ‘gross efficiency’, it is <1 and usually around 7-14% (≈ 10%)
• However, the generalization of energy transfer can misleading… why?
Energy consumed by trophic level nEnergy consumed by trophic level n-1
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Trophic Structure:energy flow
• Again, it can be the differences (e.g. between levels or among similar levels) than can provide insight into different mechanics of systems
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Trophic Structure:energy flow
• Table 1.2
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Trophic Structure:energy flow
• 3) the rate at which energy moves through a system can also be very enlightening (structure)
• It may stabilize unstable systems• May support a much larger and
complex system than otherwise could persist
• Consistent vs. pulses
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Trophic Structure:energy flow
• 4) Some communities may be enhanced (e.g. freshwater streams) or completely dependent upon external inputs of energy (e.g. detritus)
• The ra
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Trophic Structure:energy flow
• Variation in GPP across different terrestrial ecosystems
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Trophic Structure:energy flow
• Despite the insight trophodynamic perspectives have provided for community ecologists, they provide little insight in how communities are actually structured
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Trophic Structure:energy flow
• However, the idea of utilizing trophic levels permeates several analytical aspects of community ecology
• E.g. simple predator-prey dynamics• E.g. in guild analysis, trophic guilds (or
a variation thereof) are used• E.g. food webs are a more biologically
realistic approach to trophic dynamics